processImages: Improve error handling on missing graphicsmagick

In the special case where https://github.com/papandreou/express-processimage/blob/master/lib/getFiltersAndTargetContentTypeFromQueryString.js#L69 is hit, processImages will emit a warning suggesting the user to install graphicsmagick, tell what filters where not applied and just move on.

If another error is emitted on the filter stream it will be emitted as an error in assetgraph.